Off-stream decoking of a minor portion of on-stream thermal cracking tubes



Jan. 23, 1968 R. P. CAHN ET AL 3,365,387

OFF-STREAM DECOKING OF A MINOR PORTION OF' ON-STREAM THERMAL CRACKING TUBES Filed April 29, 196e CEF* N v N) ROBERT P. CAHN JOHN A. Kil/LEN mvENToRs DAV/D K. THURLOW PATENT ATTORNEY United States Patent Gtlce 3,365,33? Patented Jari. 23, i968 OFF-STREAM DECGKING F A MNOR POR- 'HN 0F ON-STREAM THERMAL CRACK- ING TUBES Robert P. Cahn, Millburn, and llohn A. Kivlen, Sparta,

NJ., and David K. Thnrlow, Brockenhurst, Hants, England, assignors to Esso Research and Engineering Company, a corporation of Delaware Filed Apr. 29, i966, Ser. No. 546,277 8 Claims. (Cl. 208-48) ABSTRACT 0F THE DISCLGSURE Thermal cracking of hydrocarbons in admixture with steam leads to the deposition of coke on furnace tube walls, which coke must be periodically removed to maintain `cracking efliciency; said coke can now be removed by passing, through one or more tubes, a steam and/or Water feed to decoke those tubes, while maintaining the furnace on stream.

The present invention pertains to a method of decoking the tubes of a cracking furnace and particularly to the decoking of the coils or tubes of a steam cracking furnace.

Various processes have been described in the prior art on high temperature thermal cracking or steam cracking of hydrocarbons including high boiling hydrocarbons such as residual oils and gas oils, and lower boiling hydrocarbons such as naphtha and hydrocarbon gases such as ethane, propane, etc., to produce olens such as ethylene, propylene, diolens such as butadiene, isoprene, etc., and aromatic hydrocarbons such as benzene, toluene, etc. In these processes the petroleum feed stock is vaporized, diluted with steam and cracked in a coil at a temperature of about 1200 to 1600* F. Residence times are relatively short, usually in the range of about 0.1 to 5.0 seconds whereupon the reaction products are immediately quenched to arrest further reactions and minimize loss of primary conversion products.

Steam cracking of the hydrocarbon feed stock is effected by supplying the feed stock in vaporized or substantially vaporized form in admixture with substantial amounts of steam to suitable coils in a cracking furnace. It is conventional to pass the reaction mixture through a number of parallel coils or tubes which pass through a convection section of the cracking furnace wherein hot combustion gases raise the temperature of the reaction mixture. Each coil or tube then passes through the tired or radiant section of the cracking furnace wherein a multiplicity of Aburners supply the heat necessary to bring the reactants to the desired reaction temperature and effect the desired reaction or conversion.

When hydrocarbon feed stocks are subjected to the heating conditions prevalent in a steam cracking coil furnace, coke deposits tend to form on the inner walls of the tubular members forming the cracking furnace coils. The problem of coke deposition on the inner surface of such cracking furnace cos has become a major concern in the steam cracking of hydrocarbons since such coke deposits not only interfere with heat flow through the tube walls into the stream of reactants but also with the flow of the rea-ction mixture due to reduction in the cross-sectional area of the tubes.

In the cracking section of a steam cracking furnace it is necessary to heat these tubes or coils to temperatures of the order of about 1600 to 2000 F. in order to obtain reactant temperatures within the tubes of about 1200 to 1600+ F. which are necessary to give yield patterns and conversion rates which are optimum for the production of the chemicals demanded by todays industry. The insulating effect of the coke deposits makes it necessary to operate the furnace and the tube metal at higher temperatures in order to obtain the desired cracking temperatures in the gas phase. Such higher temperatures cause more rapid deterioration of the heating coils or make necessary the use, if available, of more heat resistant and more expensive metals in such coils.

Aside from subjecting the feed stocks to the abovementioned high temperatures, it is critically important to maintain high throughput rates in order to minimize the time during which the hydrocarbons are subjected to these temperatures. It is equally important in many cases to maintain relatively low pressures, i.e. pressures just high enough to insure a rapid throughput rate, it being highly desirable to crack the hydrocarbon feed at a pressure approaching atmospheric. Accordingly, the pressure drop across the furnace, i.e. from the feed inlet to the product outlet, should be minimum.

It is clear, therefore, that coke build-up eventually requires carrying out a carbon removal cycle. Many procedures for the removal of such coke deposits have been proposed such as opening tube ends and drilling or grinding the coke deposits, treating the deposits with boiling water followed by steaming and blowing with air while applying heat externally of the tubes. Chemical processes have also been suggested as where the coke deposit is first impregnated with sulfuric acid and thereafter subjected to the action of an alkali carbonate solution to generate carbon dioxide gas Within the interstices of the coke deposits and by expansion of the generated gas, causing spalling of the deposits adhering to the walls of the cracking coils or tubes. It has also been proposed to add materials su-ch as potassium carbonate to the reaction mixture in order to reduce or remove coke deposits on thermal cracking coils.

Most of the methods previously employed or suggested have required that the normal function of the furnace and the coils or tubes for cracking of hydrocarbon materials be interrupted during the cleaning or coke removal operation. Such interruption of on-strearn time of the cracking coil produces serious economic problems in view of the temperatures involved and the time required to take the unit off-stream, effect the necessary removal of the coke deposits, and again bring the unit on-stream. Normal decoking of the furnace tubes often requires a feed outage of one to three days or even longer. In addition, the

cycling of bringing furnaces on and olf increases the wear of structural members, particularly tube supports.

It is the object of this invention to provide an improved process for thermal cracking hydrocarbon feed stocks in the presence of steam.

lt is a further object of this invention to provide an improved process for thermal cracking of hydrocarbon feed stocks in the presence of steam wherein coke deposits are removed without shutting down the cracking furnace and with only a minor reduction in production throughput of the system.

These and other objects will appear more clearly from the detailed specification and claims which follow.

It has now been found that coke deposits can be effectively removed from cracking furnace tubes by introducing steam and/or water at the inlet to one pass or tube of the cracking furnace while simultaneously reducing or eliminating normal hydrocarbon feed to that Pass. The remaining passes or tubes of the cracking furnace remain in normal service. It should be understood that it is within the scope of this invention to decoke more than one tube at a time, simultaneously or successively, so long as the total number of tubes out of service at any given time represents only a minor proportion of the total number of tubes in the cracking furnace. The quantities of steam and/or water are predetermined to meet the following criteria.

Sufcient steam and/or water are introduced to remove the Aheat normally going to the process fluid without exceeding the tube metal temperature allowances as determined by stress or oxidation limits for the tube material.

The temperature of the steam entering the section of the furnace to -be decoked must be about 700 F. or higher. lf water in introduced it must be vaporized and superheated to this temperature while steam must be merely superheated.

The mass rate of steam entering the section of the furnace to be decoked should preferably be greater than pounds per second per square foot of tube internal cross-sectional area when the tube outlet pressure is of the order of -25 p.s.i.a. Higher mass rates at constant Itemperature reduce the time required for decoking. Higher operating pressures in the furnace tubes being decoked require higher mass rates of steam for the same decoking time.

After a sufficient period of time the supply of steam and/ or water for decoking can be cut off from the decoked pass of the furnace and feed simultaneously reintroduced. With reasonably optimum steam mass rates and temperatures, a furnace pass may be decoked in l2 hours or less. After decoking of one pass of the furnace is complete, the same procedure may be used at any time deemed desirable to decoke additional passes, one at a time. As noted above, if so desired, two or more passes in a multipass furnace can be decoked in this manner simultaneously, but this does tend to detract from the major advantage of this scheme, namely, minimum upset to normal operation.

This invention may be more fully understood from the following idescription when read in conjunction with the accompanying drawing wherein the flow path of the reactants through an apparatus for thermal cracking of hydrocarbon is illustrated diagrammatically.

Referring to the drawing, the cracking furnace 10 comprises an upper, convection or preheat section 11 and a lower, cracking zone 12. Burners 13 are provided on the side walls and/or on the bottom of the furnace to supply heat. The number of burners provided is dependent upon the heat require-d and may vary considerably.

Although not shown in detail in the drawing, the furnace contains several conduits or passes in parallel. Each pass may contain a number of connected tubular members or tubes that provide a flow path through the convection section and into the cracking section. ln the drawing, one pass is shown, with the tubes in the convection section 11 designated by the numeral 15 and the cracking coils or tubes in the cracking zone 12 designated by the numeral 16. It is to `be understood that the number of conduits or tubes in the furnace is a function of the size of the furnace and is dictated solely by design considerations.

Hydrocarbon feed stock is supplied to the steam cracker via supply conduit 20 and manifold or distributor conduit 21 to the several parallel cracking conduits or passes. A control valve 22 is provided on each conduit 23 connecting the feed distributor 21 to each of the cracking conduits or tubes. Steam, or in the decoking operation, steam and/or water are supplied Kthrough inlet line 24 and valve 25 to the conduit 23. (In some cases, steam and water are supplied through separate lines and not necessarily at the identical point in the convection section.)

The reaction products are discharged from the coils or tubes 16 of the cracking furnace via conduits 26 into conduit or header 27 from which they are discharged into conduit 2S. In order to stop the cracking reaction promptly and thereby prevent or minimize side reactions, quenching agents such as higher boiling hydrocarbons and/or water are supplied through conduit 29 and control valve 30. The mixture of quenched reaction products and quenching agent is discharged via conduit 23 into fractionating tower 31. Aromatic tar product is withdrawn from the bottom of fractionating tower 31 through line 32 and product is taken overhead via line 33. Other intermediate boiling range fractions may be withdrawn as product or recycled to a higher plate in the fractionating tower as one or more reflux streams. The quench oil may be withdrawn from the fractionating tower 31 through line 34 and passed through heat exchanger 35 where it is passed in indirect heat exchange relation to the hydrocarbon feed stock for preheat thereof or to water for steam formation while cooling the quench oil to a suitable temperature for discharge through line 29 and valve 30 into the reaction product stream in line 28 as described above.

The on-stream decoking procedure requires the closing of one of the hydrocarbon feed valves 22 and the opening of the steam water valve 25. The amount of steam and Water passed through the decoking conduit 24 is adjusted so that the steam temperature inside the pass is about 700 F. or higher at the point of transition from convection tubes 15 to cracking tubes 16. When sufficient time has elapsed to allow the coke to be removed from the inside of the tubes, valve 25 is closed and valve 22 is opened. There are two indications that help show the progress of the coke removal:

Decrease in pressure drop. Decrease in tube metal temperature.

As a specific example of an operation according to the present invention, a steam cracking furnace containing ten passes or conduits was decoked by slowly backing out the hydrocarbon feed to one pass at a time while adding boiler feed water and adjusting normal steam flow. Three tests were carried out by varying the quantities of steam, water and time of decoking, and a fourth test was made with constant Water and steam rates in one `pass until a furnace shutdown some twenty hours later. On completion of the several tests, the furnace was shut down and the tubes were then removed from the furnace for examination. The following are summaries of the tests which were carried out and the condition of the tubes on examination after the furnace was shut down.

In the following tables C.O.T.=Coil outlet temperature. Crossover temp.='l`emperature at point of transition from convection tubes 15 to cracker tubes 16.

T.I..- Temperature indicator.

Test No. 1

Feed to pass 8 backed out over a 2-hour period. When steady conditions were reached, feed was returned to the pass D 6 801 nu 7 8 9 2 5 0 ew mu Is o 00000 0 0 o o 0 0 400 0 W 55777 7 7 7 7 9 9 504 8 um 1 66990 9 9 9 9 2 2 722 n u om. FM n 11 1 232 2 rl.

W m Y W 5 55506 0 u 4. 4 0 0 0 000000 0 Fh 1111 11 .11 1m 11 11 1 1 11 21 2222MM 2 mem m mm e l 1. n P 24 I o T m o wn o o Wr' 0 n 2 n n 17 n wh 31 t u 21 21 112 F e 1 u um D o d d 505 5 HF @www n 11e s vn 2221 n n n 65H1 6 1 1 0 U 1.1111111111 mp rm Ce T 53758558045 o 5577 5 1. 00009 9 O00 1 8056 444.43%3Mww444 4. 34u34". 4 1 T1. LLLLLLLLLLL 1 1 1 1 1 1 ,1 1111 1 OF Time Steady Conditions Feed Returned to Pass-End or Testl om. 1B

rge areas of Pass 8 completely free of coke were found upon examination of down. Remaining areas had coke deposits oi average thick- 8" (micrometer readings). Pass N o. 9 was used as a reference, Le. was not decoked oke deposit throughout its length having an average thickness .248' (micrometer was found when examined after shutdown. Pass was typical of ell passes not Test No. 2

Feed to pass 7 backed out using a steam-water mixture over e. Ai-minute period. This condition was held for one hour and the feed again returned to the pass 1 Feed on manual.

2 Feed out.

Norm-Large areas of the tube of Pass No. 7 were completely free o coke. Remaining areas had coke deposits of average thickness .088' (micrometer readings).

Test No. 3

Feed to pass 6 was backed out using After a further 30 minutes the steam a steam/Water mixture over a period of minutes. flow was reduced for a 10 second period to induce thermal shock in the tube. rlhis was repeated 5 minutes later. Feed was again returned to Nora-Large tube areas of Pass No. 6 were found to be completely free of coke when examined alter the furnace shutdown. Remaining areas had coke deposits of average thickness .OSS' (micrometer readings).

Test No. 4

Feed to pass 5 was backed out very quickly using a stearn/ water mixture. This condition was maintained until the furnace shutdown some hours later. Examination of the tube of pass No. 5 found it to be completely free or all coke throughout its length These tests provide quite conclusively that:

(a) Removal of coke was found to be related to the time length of decoking (Tests Nos. l, 2, 3 and 4).

(b) Complete removal of coke was possible in twenty hours (Test No. 4).

(c) It is perfectly in order to continue to run a steam cracking furnace with one pass having a steam/ water mixture only as the feed. There was no upset on the remaining nine parallel passes while one of the ten passes was being decoked.

It was further observed that there was no upset on any other equipment downstream of the cracking furnace during any of the above tests.

This invention is not to be limited by the illustration or examples since numerous variations are possible without departing from the scope of the appended claims.

What is claimed is:

1. In a process for thermally cracking hydrocarbon materials by passing the same in admixture with steam through a multiplicity of tubes arranged in parallel in a cracking furnace wherein said tubes are subjected to radiant heat sufficient to raise the temperature of the reactants within the tubes to about 1200-l600 F., the improvement which comprises taking a minor portion of said tubes off-stream by cutting out the flow of hydrocarbon feed and passing a deeoking feed selected from the group consisting of steam, water, and mixtures thereof through said tubes in sufficient amount to maintain the temperature within said tubes at essentially the same level as in the parallel tubes remaining on-stream and effect removal of coke accumulated on the interior of the off-stream tubes and thereafter returning said tubes to normal on-stream operation.

2. The process as defined in claim 1 wherein the feed is cut off and the decoking feed is supplied to only one of said parallel tubes at a time in order to remove coke therefrom without substantially reducing the conversion capacity of the cracking furnace as a whole.

3. The process as defined in claim 1 wherein the hydrocarbon feed is cut off and the decoking feed is supplied to several of said parallel tubes in succession, the total number of said tubes which are off-stream at any particular time representing only a minor portion of the total number of tubes in the cracking furnace.

4. In a process for thermally cracking hydrocarbon materials by passing the same in admixture with steam through a multiplicity of tubes arranged in parallel wherein said tubes are heated to an intermediate temperature in a convection section by contact with hot combustion gases and then subjected to radiant heat sufficient to raise the temperature of the reactants within the tubes to about 1200 to l600 F., the improvement which comprises taking a minor portion of said tubes at a given time off-stream by cutting out the flow of hydrocarbon feed and passing a decoking feed selected from the group consisting of steam, water, and mixtures thereof through said off-stream tubes in suicient amount to maintain the temperature of the steam within said off-stream tubes at essentially the same level as the reactants within the parallel tubes remaining on-stream in the radiant heating section, continuing the supply of the dccoking feed through said off-stream tubes for a period sucient to effect removal of coke from the interior of the off-stream tubes and thereafter returning said tubes to normal on-stream operation.

5. The process as defined in claim 4 wherein the temperature of the steam in the off-stream tubes is at least about 7200 F. as it passes from the portion of said tubes in the convection section into the portion in the radiant heating section.

6. The process as defined in claim 4 wherein the temperature of the steam in the off-stream tubes is at least about 700 F. as it passes from the portion of said tubes in the convection section into the portion in the radiant heating section, the mass rate of steam entering the section of the tube to be decoked being greater than l5 pounds per second per square foot of tube internal cross-sectional area when the tube outlet pressure is of the order of 20-25 p.s.i.a.

7. The process as defined in claim 4 wherein only one of said parallel tubes is taken off-stream at a time in order to remove coke therefrom without substantially reducing the conversion capacity of the cracking furnace as a whole, and the temperature of the steam in the off-stream tube is at least about 700 F. as it passes from the portion of said off-stream tube in the convection section into the portion of said off-stream tube in the radiant heating section.

8. The process as defined in claim 4 wherein only one of said parallel tubes is taken off-stream at a time in order to remove coke therefrom without substantially reducing the conversion capacity of the cracking furnace as a whole,

9 10 and the temperature ofthe steam in the oi-stream tube is References Cited at least abOllt 70%; F. at it paSSGS from the POIOII 0f UNITED STATES pATENTS said off-stream tu e in the convection section into the portion of said off-stream tube in the radiant heating sec- 2076847 4/1937 Jqhnston 20848 tion, the mass rate of steam entering the section of the 5 2289351 7/1'942 Dlxon et al' 208-48 tube to be decoked being greater than 15 pounds per sec- 2,3 72186 3/1945 Chapey 2%132 0nd per square foot of tube internal cross-sectional area 3268435 8/196'6 scum 208-48 when the tube outlet pressure is of the order of 20 25 P s i a HERBERT LEVINE, Primary Examiner. 

